Epitaxial reactors are machines used to deposit smooth and uniform monocrystalline or polycrystalline layers of materials on substrates; the substrates thus treated are used to produce electric devices (such as solar cells), electronic devices (such as MOSFETs and LEDs) and microelectronic devices (such as integrated circuits). Therefore, the quality of the layer deposited, in terms of defectiveness, uniform thickness and uniform resistivity, is extremely important and subject to increasingly strict requirements.
Substrates are very thin disks (typically in the interval from 100 μm to 1,500 μm) and with greatly variable diameter (typically in the interval from 1″=25 mm to 18″=450 mm), and can be made, for example, of silicon [SI], silicon carbide [SiC], germanium [Ge], gallium arsenide [GaAs], aluminium oxide or “sapphire” [Al2O3], gallium nitride[GaN].
The materials deposited are typically conductor and semiconductor materials, such as silicon [Si], silicon carbide [SiC], germanium [Ge], gallium arsenide [GaAs], aluminium nitride [AlN], gallium nitride [GaN].
The layer deposited and the substrate below can be made of the same material or of different materials.
The thickness of the layer deposited can be in a wide interval from a few nanometres to several millimetres; when the thickness of the layer deposited is greater than 1 mm, the deposition process is generally called “bulk growth”.
Known epitaxial reactors comprise a reaction chamber consisting in general essentially of a quartz piece; the quartz piece comprises a quartz piece portion having the shape of a cylinder, prism, cone, pyramid or parallelepiped and provided with at least one internal cavity defined by walls; this cavity comprises a reaction and deposition zone of the epitaxial reactor; this zone is adapted to house at least one susceptor to be heated therein; the susceptor serves to support and often also to heat the substrates.
There are reactors of many types; depending on the type, the chamber can be arranged vertically or horizontally (rarely obliquely); depending on the type, the susceptor can have the shape of a disk, prism, cylinder, pyramid, cone and can be solid or hollow; depending on the type, the susceptor can be heated by means of resistors, inductors, lamps (rarely internal burners); depending on the type, the reactors can be “cold wall” or “hot wall) (these terms refer to the walls defining the space inside which the reaction and deposition takes place).
The processes of epitaxial reactors are performed at high temperatures, i.e. from several hundreds of degrees Celsius to a few thousands of degrees Celsius (for example, deposition of polycrystalline silicon is performed at temperatures typically between 450° C. and 800° C., deposition of monocrystalline silicon on silicon substrates is performed at temperatures typically between 850° C. and 1,250° C., deposition of monocrystalline silicon carbide on silicon substrates is performed at temperatures typically between 1,200° C. and 1,400° C., deposition of monocrystalline silicon carbide on silicon carbide substrates is performed at temperatures typically between 1,500° C. and 1,700° C. for “epitaxial growth” and at temperatures typically between 1,900° C. and 2,400° C. for “bulk growth”, and use a great deal of energy (tens of kW) for heating.
Therefore, an important requirement is to prevent the thermal energy generated from being released into the environment.
For this purpose, for many tens of years it has been common practice to apply a thin layer (less than 100 μm) of gold-based material to the outer surface of the reaction chamber of epitaxial reactors; this gold layer is produced by means of a certain number of painting and drying cycles (it is not easy to obtain a smooth, uniform and non-porous layer) and reflects the infrared radiation emitted by the susceptor well.
In epitaxial reactors where the susceptor is the main element that heats the substrates (for example, in epitaxial reactors with induction heating), appropriate reflection advantageously results in a reduction in the difference in temperature between the front and the back of the substrates during the growth processes.
In general, an important requirement is that the substrates are heated uniformly during the deposition process to obtain high uniformity of thickness and resistivity.
A drawback of the solution with gold layer is that, after a certain time (i.e. a few months), the gold layer becomes detached from the quartz surface of the reaction chamber—the hotter the quartz surface is, the faster the gold layer detaches, also due to the fact that thermal expansion of gold is higher than that of quartz; this phenomenon occurs even more rapidly if the reaction chamber is cooled by means of a gaseous flow (which is relatively common), also due to the mechanical action of the gaseous flow on the layer; moreover, this phenomenon is increased by traces of acids, deriving from previous wash cycles of the reaction chamber, on the surface of the reaction chamber.
Detachment of the gold layer results in an increase in consumption of electrical power of the epitaxial reactor, as part of the infrared radiation emitted by the susceptor is released into the environment.
Moreover, as detachment of the gold layer is not smooth and uniform, this also results in a reduction in the quality of the substrates grown.
Therefore, when detachment occurs, it is necessary to disassemble the reaction chamber from the epitaxial reactor, completely remove the gold layer (already partly detached), re-apply the gold layer and re-assemble the reaction chamber in the epitaxial reactor; these operations take time, are costly and can only be performed a limited number of times.
As already mentioned, the processes of the epitaxial reactors are performed in a cavity of a reaction chamber at high temperatures; therefore, it is necessary to cool the reaction chamber. However, this cooling of chamber can cause excessive and/or non-uniform cooling of the walls that define the space in which the reaction and deposition take place; this results in a reduction in the quality of the substrates grown.